Positive Electrode Active Material For Secondary Battery, And Secondary Battery Comprising The Same

ABSTRACT

In one embodiment, a positive electrode active material for a secondary battery, the positive electrode active material being a primary particle having a monolithic structure that includes a lithium composite metal oxide of Formula 1 below, wherein the primary particle has an average particle size (D50) of 2 μm to 20 μm and a Brunauer-Emmett-Teller (BET) specific surface area of 0.15 m2/g to 0.5 m2/g, and wherein the positive electrode active material has a rolling density of 3.0 g/cc or higher under a pressure of 2 ton·f:LiaNi1-x-yCoxM1yM3zM2wO2  [Formula 1]in Formula 1, M1 is at least one selected from the group consisting of Al and Mn, M2 is any one or two or more elements selected from the group consisting of Zr, Ti, Mg, Ta, and Nb, M3 is any one or two or more elements selected from the group consisting of W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0.005≤z≤0.01, 0≤w≤0.04, 0&lt;x+y≤0.7.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/760,111, filed on Mar. 14, 2018, which is a national phase entryunder 35 U.S.C. § 371 of International Application No. PCT/KR2016/014004filed Nov. 30, 2016, published in Korean, which claims priority fromKorean Patent Application No. 10-2015-0168679, filed Nov. 30, 2015 andKorean Patent Application No. 10-2016-0161896, filed Nov. 30, 2016, allof which are incorporated herein in their entirety by reference.

TECHNICAL FIELD

The present invention relates to a positive electrode active materialfor a secondary battery that has a stable monolithic structure, therebybeing capable of improving high-temperature stability and capacitycharacteristic of a battery, and a secondary battery including the same.

BACKGROUND ART

As technology development and demand for mobile devices increase, demandfor secondary batteries as energy sources is sharply increasing. Amongthe secondary batteries, a lithium secondary battery that has highenergy density and voltage, a long cycle lifespan, and a lowself-discharge rate is commercialized and being widely used.

However, the lithium secondary battery has a problem in that its lifesharply decreases as charge and discharge are repeated. Particularly,such a problem is more serious at high temperature. This is a phenomenonthat occurs due to decomposition of an electrolyte, deterioration of anactive material, and an increase in an internal resistance of a batterydue to moisture in the battery or other reasons.

Accordingly, a positive electrode active material for a lithiumsecondary battery that is currently being vigorously researched,developed, and used is LiCoO₂ having a layered structure. AlthoughLiCoO₂ is used the most due to its excellent lifespan characteristicsand charge/discharge efficiency, there is a limitation for LiCoO₂ to beapplied to a technology for increasing battery capacity due to its lowstructural stability.

As a positive electrode active material for substituting for LiCoO₂,various lithium transition metal oxides such as LiNiO₂, LiMnO₂, LiMn₂O₄,LiFePO₄, or Li(Ni_(x1)Co_(y1)Mn_(z1))O₂ have been developed. Amongthese, LiNiO₂ has an advantage of exhibiting a high discharge capacityas a battery characteristic. However, LiNiO₂ has problems in thatsynthesis is difficult with a simple solid state reaction and thermalstability and cycle characteristics are low. Also,lithium-manganese-based oxides such as LiMnO₂ and LiMn₂O₄ haveadvantages including excellent thermal stability and low cost. However,the lithium-manganese-based oxides have problems including low capacityand low high-temperature characteristics. Particularly, LiMn₂O₄ iscommercialized in some low-cost products but has an inferior lifespancharacteristic due to structural deformation (Jahn-Teller distortion)caused by Mn. Also, although a large amount of research is currentlybeing carried out on LiFePO₄ for use in hybrid electric vehicles (HEVs)due to low cost and excellent stability, it is difficult for LiFePO₄ tobe applied to other fields due to low conductivity.

Due to such circumstances, a material that is currently beingspotlighted the most as a positive electrode active material forsubstituting for LiCoO₂ is a lithium-nickel-manganese-cobalt-basedoxide, Li(Ni_(x2)Co_(y2)Mn_(z2))O₂ (here, x2, y2, and z2 are atomicfractions of independent oxide-forming elements, and 0<x2≤1, 0<y2≤1,0<z2≤1, 0<x2+y2+z2≤1). This material has advantages in that the materialis less expensive than LiCoO₂ and can be used at high capacity and highvoltage but has disadvantages in that a rate capability and lifespancharacteristic at high temperature are inferior.

Generally, a positive electrode active material may have a form of asecondary particle in which small primary particles are condensed.However, in the case of the positive electrode active material that isin the above form of a secondary particle, lithium ions may react withmoisture, CO₂, or the like in the air while moving to a surface of theactive material and may easily form surface impurities such as Li₂CO₃and LiOH. Because the surface impurities formed in such a way decreasebattery capacity or are decomposed in a battery and generate gas andcause a swelling phenomenon, there is a serious problem inhigh-temperature stability.

With increasing demand for high capacity secondary batteries nowadays,there is a growing need for development of a positive electrode activematerial that is suitable for high capacity and capable of exhibitinghigh-temperature stability due to reduction of surface impurities.

DISCLOSURE Technical Problem

A first technical object of the present invention is to provide apositive electrode active material for a secondary battery that has astable monolithic structure, thereby being capable of improvinghigh-temperature stability and capacity characteristics of a battery,and a fabrication method thereof.

A second technical object of the present invention is to provide apositive electrode for a secondary battery, a lithium secondary battery,a battery module, and a battery pack including the positive electrodeactive material.

Technical Solution

To achieve the above objects, according to an embodiment of the presentinvention, there is provided a positive electrode active material for asecondary battery, the positive electrode active material being aprimary particle having a monolithic structure that includes a lithiumcomposite metal oxide of Formula 1 below, wherein the primary particlehas an average particle size (D₅₀) of 2 μm to 20 μm and aBrunauer-Emmett-Teller (BET) specific surface area of 0.15 m²/g to 1.9m²/g.

Li_(a)Ni_(1-x-y)Co_(x) M1_(y) M3_(z) M2_(w)O₂  [Formula 1]

(In Formula 1, M1 is at least one selected from the group consisting ofAl and Mn, M2 is any one or two or more elements selected from the groupconsisting of Zr, Ti, Mg, Ta, and Nb, M3 is any one or two or moreelements selected from the group consisting of W, Mo, and Cr, and1.0≤a≤1.5, 0<x≤0.5, 0≤y≤0.5, 0.002≤z≤0.03, 0≤w≤0.04, 0<x+y≤0.7)

According to another embodiment of the present invention, there isprovided a method of fabricating the above-described positive electrodeactive material for a secondary battery, the method including a step ofpreparing a precursor by mixing a nickel raw material, a cobalt rawmaterial, and an M1 raw material (here, M1 is at least one elementselected from the group consisting of Al and Mn) and then performing areaction, a step of mixing the precursor with a lithium raw material andan M3 raw material (here, M3 is any one or two or more elements selectedfrom the group consisting of W, Mo, and Cr) such that a molar ratio ofLi/Me (Me=the sum of metal elements in the precursor and the element M3)is 2.0 or higher and then sintering at 700° C. to 900° C. in thepresence of a boron-based sintering additive, and a step of washing aproduct obtained by a result of the sintering such that a molar ratio ofLi/Me′ (Me′=the sum of metal elements, excluding lithium, in thepositive electrode active material) in the finally fabricated positiveelectrode active material is from 1.0 to 1.5 and then drying at 150° C.to 400° C.

According to still another embodiment of the present invention, thereare provided a positive electrode for a secondary battery, a lithiumsecondary battery, a battery module, and a battery pack including thepositive electrode active material.

Other details of the embodiments of the present invention are includedin the detailed description below.

Advantageous Effects

According to the present invention, a positive electrode active materialfor a secondary battery has a monolithic structure and thus maintains astable crystal structure even during charging and discharging such thatthere is no concern about a sharp decrease in capacity due to a changein the crystal structure and the generation of surface impurities isminimized. Accordingly, excellent high-temperature stability andcapacity characteristic can be exhibited when the positive electrodeactive material is applied to a battery.

DESCRIPTION OF DRAWINGS

Because the following drawings attached to the present specificationillustrate exemplary embodiments of the present invention and serve tofacilitate understanding of the technical idea of the present inventiontogether with the above-described content of the invention, the presentinvention should not be limitedly interpreted on the basis of thedrawings.

The FIGURE is a photograph of a positive electrode active materialfabricated in Example 1-1 observed with a scanning electron microscope.

BEST MODE

Hereinafter, the present invention will be described in more detail toassist understanding of the present invention.

Terms or words used in the present specification and claims are not tobe limitedly interpreted as general or dictionary meanings and should beinterpreted as meanings and concepts that are consistent with thetechnical idea of the present invention on the basis of the principlethat an inventor may properly define concepts of terms to describe hisor her invention in the best way.

According to an embodiment of the present invention, a positiveelectrode active material for a secondary battery is a primary particlehaving a monolithic structure that includes a lithium composite metaloxide of Formula 1 below and has an average particle size (D₅₀) of 2 μmto 20 μm and a Brunauer-Emmett-Teller (BET) specific surface area of0.15 m²/g to 1.9 m²/g.

Li_(a)Ni_(1-x-y)Co_(x) M1_(y) M3_(z) M2_(w)O₂  [Formula 1]

(In Formula 1, M1 is at least one selected from the group consisting ofAl and Mn, M2 is any one or two or more elements selected from the groupconsisting of Zr, Ti, Mg, Ta, and Nb, M3 is any one or two or moreelements selected from the group consisting of W, Mo, and Cr, and1.0≤a≤1.5, 0≤x≤0.5, 0≤y≤0.5, 0.002≤z≤0.03, 0≤w≤0.04, 0<x+y≤0.7)

A composition of the lithium composite metal oxide of Formula 1 above isan average composition of the entire active material.

In the present invention, “monolithic structure” refers to a structurein which particles are in a morphological phase and are present in anindependent phase in which the particles are not condensed with eachother. An example of a particle structure opposite to the monolithicstructure may include a structure in which small-sized particles(“primary particles”) are physically and/or chemically condensed andform a relatively large-sized particle (“secondary particle”).

In this way, because the positive electrode active material according tothe present invention has a monolithic structure, a movement path oflithium ions until the lithium ions reach a surface of the positiveelectrode active material becomes long. Accordingly, surface impuritiesformed as a result of the lithium ions being moved to the surface of theactive material, reacting with moisture, CO₂, or the like in the air,and causing Li₂CO₃, LiOH, or the like to be adsorbed onto an oxidesurface may be minimized. Also, numerous problems that may occur due tosurface impurities, that is, a decrease in battery capacity, an increasein interfacial resistance due to interference with the movement oflithium ions, generation of gas due to decomposition of impurities, anda swelling phenomenon of a battery due to the above, may be prevented.As a result, a capacity characteristic, high-temperature stability, anda charge/discharge characteristic may be improved when the positiveelectrode active material is applied to a battery.

Further, because the positive electrode active material according to anembodiment of the present invention includes the lithium composite metaloxide of Formula 1, the positive electrode active material has excellentstructural stability and may improve a lifespan characteristic of abattery.

In the lithium composite metal oxide of Formula 1, Li may be included ata content corresponding to a, that is, 1.0≤a≤1.5. When a is less than1.0, there is a concern that the capacity may be deteriorated, and whena exceeds 1.5, the particle is sintered in a sintering process and itmay be difficult to fabricate an active material. In consideration ofthe balance between the remarkableness of the effect of improving acapacity characteristic of the positive electrode active material inaccordance with control of Li content and sinterability when fabricatingthe active material, Li may be included at a content of, morespecifically, 1.0≤a≤1.15.

In the lithium composite metal oxide of Formula 1, Ni may be included ata content corresponding to 1-x-y, that is, a content of 1-x-y and, forexample, may be included at a content of 0.3≤1-x-y<1. In considerationof the remarkableness of the effect of improving the capacitycharacteristic in accordance with including Ni, the Ni may be includedat a content of, more specifically 0.3≤1-x-y≤0.6 or 0.6≤1-x-y<1.

In the lithium composite metal oxide of Formula 1, Co may be included ata content corresponding to x, that is, a content of 0<x≤0.5. When x is0, there is a concern that the capacity characteristic may bedeteriorated, and when x exceeds 0.5, there is a concern that cost maybe increased. In consideration of the remarkableness of the effect ofimproving the capacity characteristic in accordance with including Co,the Co may be included at a content of, more specifically, 0.10≤x<0.35.

In the lithium composite metal oxide of Formula 1, M1 may be at leastone selected from the group consisting of Al and Mn. When the M1 is Al,the lifespan characteristic of a battery may be improved by allowing anaverage oxidation number of the active material to be maintained. Whenthe M1 is Mn, safety of the battery may be improved by improvingstructural stability of the active material.

M1 may be included at a content corresponding to y, that is, a contentof 0≤y≤0.5. When y exceeds 0.5, there is a concern that the outputcharacteristic and the capacity characteristic of the battery may ratherbe deteriorated. In consideration of the remarkableness of the effect ofimproving a battery characteristic in accordance with including theelement M1, the M1 may be included at a content of, more specifically,0.10≤y≤0.3.

In the lithium composite metal oxide of Formula 1, M3 is an elementcorresponding to Group 6 (group VIB) of the periodic table and serves tosuppress particle growth during the sintering process in fabrication ofan active material particle. In a crystal structure of the positiveelectrode active material, M3 may substitute for some of Ni, Co, or M1and may be present at a position at which Ni, Co, or M1 should bepresent or may react with lithium and form a lithium oxide. Accordingly,the crystal grain size may be controlled by adjusting a content and aninput timing of M3. Specifically, the M3 may be any one or two or moreelements selected from the group consisting of W, V, Nb, Nd, and Mo, andmore specifically, may be at least element of W and Nb. Among these, theeffect of improving the output characteristic is excellent when M3 is W,and the effect of improving high-temperature durability is superior whenM3 is Nb.

The M3 may be included at a content corresponding to z of the lithiumcomposite metal oxide of Formula 1, that is, a content of 0.002≤z≤0.03.When z is less than 0.002, it is not easy to realize an active materialthat satisfies the above-mentioned characteristics. As a result, theeffect of improving output and lifespan characteristics may beinsignificant. When z exceeds 0.03, distortion or collapse of thecrystal structure may be caused. Also, battery capacity may bedeteriorated by interfering with movement of lithium ions. Inconsideration of realization of a particle structure in accordance withcontent control of the element M3 and the remarkableness of the effectof improving a battery characteristic in accordance with therealization, M3 may be included at a content of, more specifically,0.005≤z≤0.01.

Further, the lithium composite metal oxide of Formula 1 or the elementsNi, Co, and M1 in the lithium composite metal oxide may be partiallysubstituted or doped with another element, i.e., M2, to improve abattery characteristic by controlling a distribution of metal elementswithin the active material. Specifically, M2 may be any one or two ormore elements selected from the group consisting of Zr, Ti, Mg, Ta, andNb, and more specifically, may be Ti or Mg.

The element M2 may be included at an amount corresponding to w within arange not deteriorating characteristics of the positive electrode activematerial, that is, a content of 0≤w≤0.04, specifically, at a content of0≤w≤0.02.

In the positive electrode active material, at least one metal element ofnickel, M1, and cobalt included in the lithium composite metal oxide ofFormula 1 may exhibit a concentration gradient that increases ordecreases in the active material.

In the present invention, a concentration gradient or a concentrationprofile of a metal element refers to a graph showing a content of ametal element within a depth from a particle surface to a centralportion when the x-axis indicates the depth from the particle surface tothe central portion and the y-axis indicates a content of a metalelement. For example, a positive average slope of a concentrationprofile signifies that a relatively larger amount of a correspondingmetal element is located at a particle central portion section than at aparticle surface portion, and a negative average slope thereof signifiesthat a relatively larger amount of a metal element is located at theparticle surface portion than at the particle central portion section.According to the present invention, a concentration gradient and aconcentration profile of a metal in the active material may be confirmedusing methods such as x-ray photoelectron spectroscopy (XPS) (alsoreferred to as electron spectroscopy for chemical analysis (ESCA)), anelectron probe micro analyzer (EPMA), an inductively coupledplasma-atomic emission spectrometer (ICP-AES), or time-of-flightsecondary ion mass spectrometry (ToF-SIMS), and specifically, when aprofile of a metal element in the active material is confirmed using theXPS, an atomic ratio of a metal may be measured for each etching timewhile etching the active material in a direction from a particle surfacetoward a central portion, and a concentration profile of the metalelement may be confirmed from this.

Specifically, at least one metal element of nickel, cobalt, and M1 mayhave a concentration gradient in which the concentration of the metalcontinuously changes throughout the active material particle, and theslope of the concentration gradient of the metal element may exhibit oneor more values. By having a continuous concentration gradient in thisway, because a sharp phase boundary region is not present from thecenter to the surface, a crystal structure is stabilized, and thermalstability is increased. When the slope of the concentration gradient ofa metal is constant, the effect of improving structural stability may befurther improved. Also, by varying a concentration of each of the metalswithin the active material particle by a concentration gradient,characteristics of a corresponding metal can be easily utilized, and theeffect of the positive electrode active material improving batteryperformance can be further improved.

In the present invention, “a concentration of a metal exhibits acontinuously-changing concentration gradient” means that a concentrationof a metal has a gradually-changing concentration distributionthroughout the active material particle. Specifically, in theconcentration distribution, a change in metal concentration per 1 μm,more specifically, 0.1 μm in a particle may be a difference of 0.1 atom% to 30 atom %, more specifically, 0.1 atom % to 20 atom %, and evenmore specifically, 1 atom % to 10 atom %, based on a total atomic weightof a corresponding metal included in the precursor.

More specifically, a concentration of nickel included in the activematerial may decrease with a continuous concentration gradient in adirection from a center of the active material particle toward a surfaceof the particle. Here, a slope of the concentration gradient of nickelmay be constant from the center of the active material particle towardthe surface. In this way, when the concentration of nickel remains highat the center of the active material particle and the concentration ofnickel includes a concentration gradient that gradually decreases towardthe surface of the active material, thermal stability of the positiveelectrode active material may be improved.

A concentration of M1 included in the active material may increase witha continuous concentration gradient in a direction from the center ofthe active material particle toward the surface of the particle. Here, aslope of the concentration gradient of M1 may be constant from thecenter of the active material particle toward the surface. In this way,when the concentration of M1, in particular, manganese, remains low atthe center of the active material particle and the concentration of M1includes a concentration gradient that gradually increases toward thesurface of the active material, thermal stability of the positiveelectrode active material may be improved without a decrease in capacityof the positive electrode active material. More specifically, the M1 maybe Mn.

A concentration of cobalt included in the active material may increasewith a continuous concentration gradient in a direction from the centerof the active material particle toward the surface of the particle.Here, a slope of the concentration gradient of cobalt may be constantfrom the center of the active material particle toward the surface. Inthis way, when the concentration of cobalt remains low at the center ofthe active material and the concentration of cobalt includes aconcentration gradient that gradually increases toward the surface, acapacity characteristic of the positive electrode active material may beimproved while reducing a use amount of cobalt.

Nickel, M1, and cobalt, may independently exhibit a concentrationgradient that changes throughout the active material particle, theconcentration of nickel may decrease with a continuous concentrationgradient in the direction from the center of the active materialparticle toward the surface thereof, and the concentrations of cobaltand M1 may independently increase with a continuous concentrationgradient in the direction from the center of the active material towardthe surface thereof. In this way, by including a combined concentrationgradient, in which the concentration of nickel decreases and theconcentrations of cobalt and M1 increase toward the surface of theactive material throughout the active material, thermal stability may beimproved while a capacity characteristic of the positive electrodeactive material is maintained.

According to an embodiment of the present invention, the positiveelectrode active material may have a polyhedral shape by controlling acontent and an input timing of the element M3 and a heat treatmentcondition in the fabrication process.

Due to such a unique shape, the positive electrode active material mayfacilitate intercalation and deintercalation of lithium ions, and thelithium ions may move at a high speed even in the active materialparticle, thereby exhibiting a further improved output characteristicwhen the positive electrode active material is applied to a battery.More specifically, the positive electrode active material may have arectangular parallelepiped shape or plate shape whose cross-sectionincluding a long axis that passes through the center of the particle isrectangular.

The positive electrode active material according to an embodiment of thepresent invention having the above structure and configuration may havean average particle size (D₅₀) of 2 μm to 20 μm and aBrunauer-Emmett-Teller (BET) specific surface area of 0.15 m²/g to 1.9m²/g.

Generally, although an output characteristic is improved as the BETsurface is larger because an ion contact surface is larger, anopportunity for lithium ions to move to a surface of an active materialand react with moisture, CO₂, or the like in the air increases due tothe large BET surface, and there is a problem in which the possibilityof impurities being adsorbed onto the surface increases. When theparticle size of the active material particle is too small,reversibility of charge/discharge may be deteriorated because crystalsare not well-developed, and a problem in which a condensed phase may beformed occurs. However, there is a limitation in increasing a particlesize in the fabrication process of an oxide, and when an averageparticle size is too large, efficiency of a battery with respect toweight may be deteriorated, a relatively small amount of the positiveelectrode active material may be included with respect to the same size,or applicability of the active material particle may be deteriorated andthe positive electrode active material may be missing in a fabricationprocess of an electrode, thereby deteriorating battery capacity.

With respect to this, the positive electrode active material accordingto an embodiment of the present invention simultaneously satisfies theaverage particle size and BET specific surface area conditions, therebysignificantly decreasing the adsorption of surface impurities andexhibiting an excellent output characteristic despite a small ioncontact area. More specifically, the positive electrode active materialmay have an average particle size (D₅₀) of 2 μm to 8 μm and a BETspecific surface area of 0.15 m²/g to 0.5 m²/g.

In the present invention, the average particle size (D₅₀) of the activematerial may be defined as a particle size based on a particle sizedistribution at 50%. According to the present invention, the averageparticle size (D₅₀) of the active material may be measured using, forexample, electron microscopy observation using scanning electronmicroscopy (SEM), field emission scanning electron microscopy (FE-SEM),or the like or using a laser diffraction method. More specifically, whenthe average particle size (D₅₀) is measured using the laser diffractionmethod, the positive electrode active material particles may bedispersed in a dispersion medium, the dispersed particles may beintroduced into a commercially available laser diffraction particle sizemeasurement device (for example, Microtrac MT 3000), and then anultrasonic wave of about 28 kHz may be radiated with an output of 60 Wto calculate the average particle size (D₅₀) based on a particle sizedistribution at 50% in the measurement device. According to the presentinvention, the specific surface area of the positive electrode activematerial is measured using a BET method. Specifically, the specificsurface area may be calculated from a nitrogen gas absorption amount ata liquid nitrogen temperature (77K) using BELSORP-mini II of BEL Japancompany.

According to an embodiment of the present invention, the positiveelectrode active material may have a particle size distribution value(Dcnt), which is defined by Equation 1 below, of 0.5 to 1.0, morespecifically, 0.55 to 0.9. When the particle size distribution value ofthe active material is less than 0.5, there is a concern that a processof fabricating a high-density electrode may be difficult, and when theparticle size distribution value exceeds 1.0, there is a concern thatrolling processability may be deteriorated.

Dcnt=[Dn90−Dn10]/Dn50  [Equation 1]

(In Equation 1 above, Dn90, Dn10, and Dn50 are number average particlesizes of the active material measured under 90%, 10%, and 50%,respectively, in an absorption mode using a Microtrac particle sizeanalyzer after the active material is left in distilled water for 3hours) According to an embodiment of the present invention, the positiveelectrode active material has the average particle size and specificsurface area in the above ranges, thereby exhibiting a high rollingdensity of 3.0 g/cc or higher or 3.0 g/cc to 4.5 g/cc under a pressureof 2 ton·f.

By having a high rolling density in the above range, capacity per unitvolume may increase and a high-temperature storage characteristic mayalso be improved. Also, compared to when a conventional operatingvoltage range is from 3.0 V to 4.35 V, the operating voltage range isexpanded to 2.50 V to 4.35 V, and a discharge end voltage is loweredsuch that capacity can be maximized. According to the present invention,a rolling density of the positive electrode active material may bemeasured using a general rolling density measuring device, specifically,a powder resistivity characteristic measuring device (HPRM-A1, Han TechCompany Ltd.). The rolling density may be calculated using a density ofa pellet that is formed by filling an insulating mold with powder andapplying a pressure in a vertical direction. The rolling density isaffected by a crystal grain size and a degree of particle condensation.

According to an embodiment of the present invention, the positiveelectrode active material may be fabricated by a fabrication methodincluding a step of preparing a precursor by mixing a nickel rawmaterial, a cobalt raw material, and an M1 raw material (here, M1 is atleast one element selected from the group consisting of Al and Mn) andthen performing a reaction (Step 1), a step of mixing the precursor witha lithium raw material and an M3 raw material (here, M3 is any one ortwo or more elements selected from the group consisting of W, Mo, andCr) such that a molar ratio of Li/Me (Me=the sum of metal elements inthe precursor and the element M3) is 2.0 or higher and then sintering at700° C. to 900° C. in the presence of a boron-based sintering additive(Step 2), and a step of washing a product obtained by a result of thesintering such that a molar ratio of Li/Me′ (Me′=the sum of metalelements excluding lithium) in the finally fabricated positive electrodeactive material is from 1.0 to 1.5 and then drying at 150° C. to 400° C.(Step 3). Here, when the positive electrode active material furtherincludes M2 (here, M2 is any one or two or more elements selected fromthe group consisting of Zr, Ti, Mg, Ta, and Nb), an M2 raw material maybe added during the mixing of the raw materials of the metal elements inStep 1, or the M2 raw material may be added during the mixing with thelithium raw material in Step 2. Accordingly, according to anotherembodiment of the present invention, a method of fabricating theabove-described positive electrode active material is provided.

Hereinafter, each of the steps will be described in detail. In thefabrication method for fabricating the positive electrode activematerial, Step 1 is a step of preparing a precursor by using the nickelraw material, the cobalt raw material, the M1 raw material andselectively using the M2 raw material.

Specifically, the precursor may be fabricated by adding an ammoniumcation-containing complexing agent and a basic compound to ametal-containing solution, which is produced by mixing the nickel rawmaterial, the cobalt raw material, the M1 raw material, and the M2 rawmaterial, and performing a coprecipitation reaction. Here, a mixingratio of the raw materials may be properly determined within the rangethat allows the conditions of contents of the metal elements in thefinally fabricated positive electrode active material to be satisfied.

The metal-containing solution may be produced by adding the nickel rawmaterial, the cobalt raw material, the M1-containing raw material, andselectively, the M2-containing raw material to a solvent, specifically,water or a mixture of water and an organic solvent (specifically, analcohol or the like) that may be uniformly mixed with water.Alternatively, solutions, specifically, aqueous solutions, including theraw materials may be produced, the solutions may be mixed, and then themixture may be used as the metal-containing solution.

An acetate, a nitrate, a sulfate, a halide, a sulfide, a hydroxide, anoxide, an oxyhydroxide, or the like may be used as the metalelement-containing raw material, and the metal element-containing rawmaterial is not particularly limited as long as the metalelement-containing raw material can be dissolved in water.

For example, examples of the cobalt raw material may include Co(OH)₂,CoOOH, Co(SO₄)₂, Co(OCOCH₃)₂.4H₂O, Co(NO₃)₂.6H₂O, COCl₂, Co(SO₄)₂.7H₂O,or the like, and any one of the above or a mixture of two or more of theabove may be used as the cobalt raw material.

Examples of the nickel raw material may include Ni(OH)₂, NiO, NiOOH,NiCO_(3.).2Ni(OH)₂.4H₂O, NiC₂O₂.2H₂O, NiCl₂, Ni(NO₃)₂.6H₂O, NiSO₄,NiSO₄.6H₂O, fatty acid nickel salts, nickel halides, or the like, andany one of the above or a mixture of two or more of the above may beused as the nickel raw material.

Examples of the manganese raw material may include manganese oxides suchas Mn₂O₃, MnO₂, and Mn₃O₄; manganese salts such as MnCO₃, MnCl₂,Mn(NO₃)₂, MnSO₄, manganese acetate, manganese dicarboxylate, manganesecitrate, and fatty acid manganese salts; an oxyhydroxide, manganesechloride, and the like, and any one of the above or a mixture of two ormore of the above may be used as the manganese raw material.

Examples of an aluminum raw material may include AlSO₄, AiCl₃,Al-isopropoxide, AlNO₃, or the like, and any one of the above or amixture of two or more of the above may be used as the aluminum rawmaterial.

An acetate, a nitrate, a sulfate, a halide, a sulfide, a hydroxide, anoxide, an oxyhydroxide, or the like including the element M2 may be usedas the M2 raw material. For example, when M2 is Ti, a titanium oxide maybe used.

The ammonium cation-containing complexing agent may be, specifically,NH₄OH, (NH₄)₂SO₄, NH₄NO₃, NH₄Cl, CH₃COONH₄, NH₄CO₃, or the like, and anyone of the above or a mixture of two or more of the above may be used asthe ammonium cation-containing complexing agent. Further, the ammoniumcation-containing complexing agent may also be used in the form of anaqueous solution, and here, water or a mixture of water and an organicsolvent (specifically, an alcohol or the like) that may be uniformlymixed with water may be used as a solvent.

The ammonium cation-containing complexing agent may be added in anamount such that the molar ratio is 0.5 to 1 with respect to 1 mol ofthe metal-containing solution. Generally, a chelating agent reacts witha metal at a molar ratio of 1:1 or higher to form a complex. However,because an unreacted complex in the formed complex that has not reactedwith the basic aqueous solution may be changed into an intermediateproduct and be recovered as a chelating agent for reuse, the use amountof chelating agent may be lowered in the present invention in comparisonto the general case. As a result, the crystallinity of the positiveelectrode active material may be improved, and the positive electrodeactive material may be stabilized.

The basic compound may be a hydroxide of an alkali metal or an alkalineearth metal such as NaOH, KOH, Ca(OH)₂, or the like or a hydratethereof, and any one of the above or a mixture of two or more of theabove may be used as the basic compound. The basic compound may also beused in the form of an aqueous solution, and here, water or a mixture ofwater and an organic solvent (specifically, an alcohol or the like) thatmay be uniformly mixed with water may be used as a solvent.

The coprecipitation reaction for forming the precursor may be performedunder a condition in which pH is 11 to 13. When the pH deviates from theabove range, there is a concern that the size of the fabricatedprecursor may be changed or particle fragmentation may be caused. Also,there is a concern that metal ions may be eluted at a surface of theprecursor and form various oxides by side reaction. More specifically,the coprecipitation reaction may be performed under a condition in whichpH of a mixture is 11 to 12.

The ammonium cation-containing complexing agent and the basic compoundmay be used in a molar ratio of 1:10 to 1:2 to satisfy theabove-mentioned pH range. Here, the pH value refers to a pH value of aliquid at a temperature of 25° C.

The coprecipitation reaction may be performed at a temperature of 40° C.to 70° C. under an inert atmosphere of nitrogen or the like. Further, astirring process may be selectively performed to increase a reactionspeed during the reaction, and here, the stirring speed may be 100 rpmto 2,000 rpm.

When a concentration gradient of a metal element in the finallyfabricated positive electrode active material is attempted to be formed,a metal-containing solution (hereinafter simply referred to as a secondmetal-containing solution) including nickel, cobalt, M1-containing rawmaterial and, selectively, M2-containing raw material in differentconcentrations from the above-described metal-containing solution may beprepared, then, simultaneously, the second metal-containing solution maybe added to the above-described metal-containing solution such that amixing ratio of the above metal-containing solution and the second-metalcontaining solution is gradually changed from 100 vol %:0 vol % to 0 vol%:100 vol %, and the ammonium cation-containing complexing agent and thebasic compound may be added thereto, and a reaction may be performed,thereby forming the concentration gradient.

In this way, by continuously increasing an amount of the secondmetal-containing solution being input into the above-describedmetal-containing solution and controlling a reaction speed and reactiontime, a precursor that exhibits a concentration gradient in whichconcentrations of nickel, cobalt, and M1 independently continuouslychange from the center of the particle toward a surface thereof may befabricated with a single coprecipitation reaction process. In this case,the concentration gradient of a metal in the resulting precursor and aslope of the concentration gradient may be easily adjusted bycompositions and mixture supply ratios of the above-describedmetal-containing solution and the second metal-containing solution.Extending a reaction time and decreasing a reaction speed are preferableto reach a high density state in which a concentration of a specificmetal is high, and shortening a reaction time and increasing a reactionspeed are preferable to reach a low density state in which aconcentration of a specific metal is low.

Specifically, the second metal-containing solution may be added to theabove-described metal-containing solution such that a speed of addingthe second metal-containing solution is continuously increased withinthe range of 1% to 30% of an initial input speed. Specifically, an inputspeed of the above-described metal-containing solution may be 150 ml/hrto 210 ml/hr, an input speed of the second metal-containing solution maybe 120 ml/hr to 180 ml/hr, and the input speed of the secondmetal-containing solution may be continuously increased within the rangeof 1% to 30% of the initial input speed within the input speed range.Here, the reaction may be performed at 40° C. to 70° C. Also, a size ofa precursor particle may be adjusted by adjusting a supply amount andreaction time of the second metal-containing solution with respect tothe above-described metal-containing solution.

By the above process, a particle of a composite metal hydroxide isgenerated and precipitated in a reaction solution as a precursor.Specifically, the precursor may include a compound of Formula 2 below.

Ni_(1-x-y)Co_(x) M1_(y) M3_(z) M2_(w)(OH_(1-a))₂  [Formula 2]

(In Formula 2, M1, M2, M3, x, y, z, and w are the same as those definedabove, and 0≤a≤0.5)

The nickel raw material, the cobalt raw material, and the M1 rawmaterial may be metal powder including respective metal elements. Inthis case, the precursor may be fabricated by mixing the powdery rawmaterials including respective metal elements and heat treating themixture at 200° C. to 500° C.

Next, in the fabrication method for fabricating the positive electrodeactive material, Step 2 is a step of forming the lithium composite metaloxide of Formula 1 by mixing the precursor particle fabricated in Step 1with the lithium raw material, the M3 raw material, and selectively withthe M2 raw material and then sintering. Here, the M2 raw material is thesame as that described above.

Examples of the lithium raw material may include a lithium-containingcarbonate (for example, lithium carbonate or the like), a hydrate (forexample, lithium hydroxide I hydrate (LiOH.H₂O) or the like), ahydroxide (for example, lithium hydroxide or the like), a nitrate (forexample, lithium nitrate (LiNO₃) or the like), and a chloride (forexample, lithium chloride (LiCl) or the like), and any one of the aboveor a mixture of two or more of the above may be used as thelithium-containing raw material. A use amount of the lithium-containingraw material may be determined in accordance with contents of lithiumand a transition metal in the finally fabricated lithium composite metaloxide, and specifically, the lithium-containing raw material may be usedin an amount such that the molar ratio between lithium included in thelithium raw material and the metal element (Me) included in thecomposite metal hydroxide (molar ratio of lithium/metal element (Me=thesum of metal elements in the precursor and the element M3) is 1.0 orhigher, more specifically, 2.0 or higher.

An acetate, a nitrate, a sulfate, a halide, a sulfide, a hydroxide, anoxide, an oxyhydroxide, or the like including the element M3 may be usedas the M3 raw material. For example, when M3 is W, a tungsten oxide maybe used. The M3 raw material may be used within the range that allows acondition of a content of the element M3 in the finally fabricatedpositive electrode active material to be satisfied.

After the precursor is mixed with the lithium-containing raw material,the M3 raw material, and selectively with the M2 raw material, apreliminary heat treatment process at 250° C. to 500° C. may beselectively performed before the sintering process. By such apreliminary heat treatment process, a sintering rate during thesubsequent sintering process may be increased.

The preliminary heat treatment process may be performed in one step ormay also be performed in multiple steps at different temperatures.

The sintering process may be performed at 700° C. to 900° C. or 750° C.to 850° C.

The shape, size, aspect ratio, and orientation of the primary particlemay be controlled by adjusting temperature during the sintering process,and the positive electrode active material having the above-describedstructure may be fabricated by performing the sintering process in theabove temperature ranges. The sintering process may also be performed inmultiple steps of two to three.

The sintering process may be performed in an air atmosphere or an oxygenatmosphere (for example, O₂ or the like), and more specifically, may beperformed under an oxygen atmosphere at an oxygen partial pressure of 20vol % or higher. The sintering process may be performed for 5 hours to48 hours or for 10 hours to 20 hours under the above conditions.

The sintering process may be performed in the presence of a sinteringadditive.

When the sintering additive is added, crystals may be easily grown atlow temperature, and non-uniform reaction may be minimized during drymixing. Specifically, examples of the sintering additive may includeboron-based compounds such as boric acid, lithium tetraborate, boronoxide, and ammonium borate, and any one or a mixture of two or more ofthe above may be used as the sintering additive. The sintering additivemay be used in an amount of 0.2 parts by weight to 2 parts by weight,more specifically, 0.4 parts by weight to 1.4 parts by weight, withrespect to 100 parts by weight of the precursor.

A moisture-removing agent may be selectively further added during thesintering process. Specifically, examples of the moisture-removing agentmay include citric acid, tartaric acid, glycolic acid, maleic acid, orthe like, and any one of the above or a mixture of two or more of theabove may be used as the moisture-removing agent. The moisture-removingagent may be used in an amount of 0.01 to 2 parts by weight with respectto 100 parts by weight of the precursor.

In the fabrication method according to an embodiment of the presentinvention, a washing process and a drying process for removingimpurities present on a surface of a reactant that is obtained as aresult of the sintering may be performed in Step 3.

The washing process may be performed in accordance with a general methodexcept that the washing process is performed so that the molar ratio ofLi/Me′ (Me′=the sum of metal elements, excluding lithium, in thepositive electrode active material) in the product obtained as theresult of the sintering is 1.0 to 1.5. Specifically, the washing processmay be performed by washing using water or a lower alcohol having acarbon number of 1 to 4.

Also, the drying process may be performed in accordance with a generaldrying method. Specifically, the drying process may be performed usingmethod such as heat treatment and hot air injection in the temperaturerange of 150° C. to 400° C., and more specifically, may be performed for15 to 30 hours at the above temperature range.

By having a monolithic structure, the positive electrode active materialfabricated by the above process can maintain a stable crystal structureeven during charge/discharge such that there is not concern about asharp decrease in a capacity due to a change in the crystal structure,and generation of surface impurities is minimized such that excellenthigh-temperature stability and capacity characteristic can be exhibitedwhen the positive electrode active material is applied to a battery.

Accordingly, according to still another embodiment of the presentinvention, a positive electrode and a lithium secondary batteryincluding the above-described positive electrode active material areprovided.

Specifically, the positive electrode includes a positive electrodecurrent collector and a positive electrode active material layer formedon the positive electrode current collector and including theabove-described positive electrode active material.

The positive electrode current collector is not particularly limited aslong as the positive electrode current collector does not cause achemical change to a battery and has conductivity, and for example,stainless steel, aluminum, nickel, titanium, calcined carbon, or analuminum or stainless steel whose surface is treated with carbon,nickel, titanium, silver, or the like may be used as the positiveelectrode current collector. Generally, the positive electrode currentcollector may have a thickness of 3 to 500 μm, and an adhesive force ofa positive electrode active material may be improved by forming fineirregularities on a surface of the current collector. For example, thepositive electrode current collector may be used in various forms suchas a film, a sheet, a foil, a net, a porous body, a foam body, and anonwoven fabric body.

In addition to the above-described positive electrode active material,the positive electrode active material layer may include a conductivematerial and a binder.

Here, the conductive material is used to impart conductivity to anelectrode, and in a constituted battery, any conductive material can beused without particular limitations as long as the conductive materialdoes not cause a chemical change and has electronic conductivity.Specific examples include graphite such as natural graphite orartificial graphite; a carbon-based material such as carbon black,acetylene black, Ketjen black, channel black, furnace black, lamp black,summer black, and carbon fiber; metal powder or metal fiber such ascopper, nickel, aluminum, and silver; conductive whisker such as zincoxide and potassium titanate; a conductive metal oxide such as titaniumoxide; a conductive polymer such as a polyphenylene derivative, or thelike, and any one or a mixture of two or more selected therefrom may beused as the conductive material. Generally, the conductive material maybe included at 1 wt % to 30 wt % with respect to a total weight of thepositive electrode active material layer.

The binder performs a role of improving adhesion between positiveelectrode active material particles and an adhesive force between thepositive electrode active material and the current collector. Specificexamples thereof include polyvinylidene fluoride (PVDF), aPVDF-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol,polyacrylonitrile, carboxymethyl cellulose (CMC), starch,hydroxypropylcellulose, regenerated cellulose, polyvinyl pyrrolidone,tetrafluoroethylene, polyethylene, polypropylene, anethylene-propylene-diene monomer (EPDM), a sulfonated EPDM, styrenebutadiene rubber (SBR), fluororubber, or various copolymers thereof, andone or a mixture of two or more selected therefrom may be used as thebinder. The binder may be included at 1 to 30 wt % with respect to thetotal weight of the positive electrode active material layer.

Except for use of the above-described positive electrode activematerial, the positive electrode may be fabricated in accordance with ageneral method of fabricating a positive electrode. Specifically, thepositive electrode may be fabricated by applying a composition forforming a positive electrode active material layer that includes theabove-described positive electrode active material and selectivelyincludes the binder and the conductive material on the positiveelectrode current collector and then drying and rolling. Here, the typesand contents of the positive electrode active material, the binder, andthe conductive material are the same as those described above.

A solvent may be a solvent that is generally used in the art, examplesof the solvent may include dimethyl sulfoxide (DMSO), isopropyl alcohol,N-methyl pyrrolidone (NMP), acetone, or water, and one or a mixture oftwo or more selected therefrom may be used as the solvent. A use amountof the solvent is sufficient as long as the solvent has a viscositycapable of allowing the positive electrode active material, theconductive material, and the binder to be dissolved or dispersed andexhibiting excellent thickness uniformity when applied layer forfabricating a positive electrode in consideration of a thickness of anapplied slurry and a fabrication yield.

With another method, the positive electrode may be fabricated by castingthe composition for forming the positive electrode active material layeron a separate support body and then laminating a film obtained byseparation from the support body on the positive electrode currentcollector.

According to still another embodiment of the present invention, anelectrochemical device including the positive electrode is provided.Specifically, the electrochemical device may be a battery, a capacitor,and the like, and more specifically, may be a lithium secondary battery.

Specifically, the lithium secondary battery includes a positiveelectrode, a negative electrode disposed opposite the positiveelectrode, a separator interposed between the positive electrode and thenegative electrode, and an electrolyte, and the positive electrode isthe same as that described above. Also, the lithium secondary batterymay selectively further include a battery container configured to storean electrode assembly including the positive electrode, the negativeelectrode, and the separator, and a sealing member configured to sealthe battery container.

In the lithium secondary battery, the negative electrode includes anegative electrode current collector and a negative electrode activematerial layer disposed on the negative electrode current collector.

The negative electrode current collector is not particularly limited aslong as the negative electrode current collector does not cause achemical change to a battery and has high conductivity, and for example,copper, stainless steel, aluminum, nickel, titanium, calcined carbon, ora copper or stainless steel whose surface is treated with carbon,nickel, titanium, silver, and the like, an aluminum-cadmium alloy etc.may be used as the negative electrode current collector. Generally, thenegative electrode current collector may have a thickness of 3 to 500μm, and, like the positive electrode current collector, an adhesiveforce of a negative electrode active material may be improved by formingfine irregularities on a surface of the current collector. For example,the negative electrode current collector may be used in various formssuch as a film, a sheet, a foil, a net, a porous body, a foam body, anda nonwoven fabric body.

In addition to the above-described negative electrode active material,the negative electrode active material layer may selectively include aconductive material and a binder. As an example, the negative electrodeactive material layer may be fabricated by applying a composition forforming a negative electrode that includes the negative electrode activematerial and selectively includes the binder and the conductive materialon the negative electrode current collector and then drying, or bycasting the composition for forming the negative electrode on a separatesupport body and then laminating a film obtained by separation from thesupport body on the negative electrode current collector.

A compound capable of reversible intercalation and deintercalation oflithium may be used as the negative electrode active material. Specificexamples include a carbonaceous material such as artificial graphite,natural graphite, graphitized carbon fiber, and amorphous carbon; ametallic compound capable of being formed into an alloy with lithium,such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, an Si-alloy, anSn-alloy, and an Al-alloy; a metal oxide capable of doping and dedopinglithium, such as SiOx (0<x<2), SnO2, vanadium oxide, and lithiumvanadium oxide; or a composite including the metallic compound and thecarbonaceous material, such as an Si—C compound or an Sn—C compound, andone or a mixture of two or more selected therefrom may be used as thenegative electrode active material. Also, a metal lithium thin film maybe used as the negative electrode active material. Both low crystallinecarbon and high crystalline carbon may be used as the carbon material.Soft carbon and hard carbon are typical low crystalline carbon, andhigh-temperature calcined carbons such as amorphous, plate-shaped,flake-shaped, spherical, or fiber type natural graphite or artificialgraphite, Kish graphite, pyrolytic carbon, mesophase pitch based carbonfiber, meso-carbon microbeads, mesophase pitches, and petroleum or coaltar pitch derived cokes are typical high crystalline carbon.

Also, the binder and the conductive material may be the same as thosedescribed above with respect to the positive electrode.

In the lithium secondary battery, the separator separates the negativeelectrode and the positive electrode and provides a movement path forlithium ions, and anything that is generally used as a separator in alithium secondary battery may be used without particular limitation.Particularly, it is preferable that the separator have low resistancewith respect to ion movement in an electrolyte and have excellentability of impregnating an electrolyte. Specifically, a porous polymerfilm, for example, a porous polymer film fabricated with apolyolefin-based polymer such as an ethylene homopolymer, a propylenehomopolymer, an ethylene-butene copolymer, an ethylene-hexene copolymer,and an ethylene-methacrylate copolymer or a stacked structure of two ormore layers thereof may be used. Also, a general porous nonwoven fabric,for example, a nonwoven fabric formed of high-melting-point glass fiber,polyethylene terephthalate fiber, or the like may be used. A coatedseparator including a ceramic component or a polymer material forsecuring heat resistance or mechanical strength may be used, and may beused in a single-layer or multi-layer structure.

Examples of an electrolyte used in the present invention may include anorganic liquid electrolyte, an inorganic liquid electrolyte, a solidpolymer electrolyte, a gel-type polymer electrolyte, an inorganic solidelectrolyte, and a molten-type inorganic electrolyte, which can be usedin the manufacture of a lithium secondary battery, but the presentinvention is are not limited thereto.

Specifically, the electrolyte may include an organic solvent and alithium salt.

Anything that can serve as a medium capable of moving ions that areinvolved in an electrochemical reaction of a battery can be used as theorganic solvent without particular limitation. Specifically, anester-based solvent such as methyl acetate, ethyl acetate,γ-butyrolactone, and ε-caprolactone; an ether-based solvent such asdibutyl ether or tetrahydrofuran; a ketone-based solvent such ascyclohexanone; an aromatic hydrocarbon-based solvent such as benzene andfluorobenzene; a carbonate-based solvent such as dimethylcarbonate(DMC), diethylcarbonate (DEC), methylethylcarbonate (MEC),ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylenecarbonate (PC); an alcohol-based solvent such as ethyl alcohol andisopropyl alcohol; nitriles such as R—CN (R is a linear, branched, orcyclic hydrocarbon group of C2 to C20, and may include a double bondaromatic ring or an ether bond); amides such as dimethylformamide;dioxolanes such as 1.3-dioxolane; or sulfolane may be used as theorganic solvent. Among these, the carbonate-based solvent is preferable,and a mixture of a cyclic carbonate (for example, EC or PC) having highion conductivity and a high dielectric constant capable of improving thecharge/discharge performance of a battery and a linear carbonate-basedcompound (for example, EMC, DMC, or DEC) having a low viscosity is morepreferable. In this case, excellent performance of an electrolyte may beexhibited when the cyclic carbonate and the chain carbonate are mixed ina volume ratio of about 1:1 to 1:9 and used.

Any compound capable of providing lithium ions used in a lithiumsecondary battery may be used as the lithium salt without particularlimitation. Specifically, LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiSbF₆, LiAlO₄,LiAlCl₄, LiCF₃SO₃, LiC₄F₉SO₃, LiN(C₂F₅SO₃)₂, LiN(C₂F₅SO₂)₂,LiN(CF₃SO₂)₂. LiCl, LiI, LiB(C₂O₄)₂, or the like may be used as thelithium salt. The concentration of the lithium salt is preferably in therange of 0.1 to 2.0 M. When the concentration of the lithium salt iswithin the above range, because the electrolyte has an appropriateconductivity and viscosity, the electrolyte can exhibit excellentelectrolyte performance, and the lithium ions can effectively move.

For purposes of improving a lifespan characteristic of a battery,suppressing a decrease in battery capacity, improving a dischargecapacity of a battery, and the like, for example, the electrolyte mayfurther include one or more additives such as haloalkylenecarbonate-based compound such as difluoroethylene carbonate or the like,pyridine, triethyl phosphite, triethanolamine, cyclic ether,ethylenediamine, n-glyme, hexaphosphate triamide, a nitrobenzenederivative, sulfur, quinone imine dyes, N-substituted oxazolidinone,N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammoniumsalt, pyrrole, 2-methoxyethanol, aluminum trichloride, or the like inaddition to the components of the electrolyte. Here, the additive may beincluded at 0.1 to 5 wt % with respect to the total weight of theelectrolyte.

Because the lithium secondary battery including the positive electrodeactive material according to the present invention stably exhibits anexcellent discharge capacity, output characteristic, and capacityretention as described above, the lithium secondary battery is usefulfor portable devices such as a mobile phone, a laptop computer, and adigital camera and in the electric vehicle field including a hybridelectric vehicle (HEV).

Accordingly, according to another implementation of the presentinvention, a battery module including the lithium secondary battery as aunit cell and a battery pack including the same are provided.

The battery module or battery pack may be used as a power source of amedium to large sized device of one or more of a power tool; an electriccar including an electric vehicle (EV), a hybrid electric vehicle, and aplug-in hybrid electric vehicle (PHEV); or a power storage system.

[Mode]

Hereinafter, an embodiment of the present invention will be described indetail so that one of ordinary skill in the art to which the presentinvention pertains can easily practice the present invention. However,the present invention may be implemented in various other forms and isnot limited to the embodiment described herein.

Example 1-1: Fabrication of Positive Electrode Active Material

In a batch-type 5 L reactor set at 60° C., nickel sulfate, cobaltsulfate, and manganese sulfate were mixed in a molar ratio of 60:20:20in water to prepare a metal-containing solution at a concentration of 2M.

A vessel containing the metal-containing solution was connected to enterthe reactor, and a 4 M NaOH solution and a NH₄OH aqueous solution at aconcentration of 7% were prepared and connected to the reactor,respectively. 3 L of deionized water was added to a coprecipitationreactor (capacity: 5 L), the reactor was purged with nitrogen gas at arate of 2 L/minute to remove oxygen dissolved in water, and anon-oxidizing atmosphere was formed in the reactor. Then, 100 ml of 4MNaOH was added thereto, and the mixture was maintained at pH 12.0 at astirring speed of 1,200 rpm at 60° C. Then, the metal-containingsolution, the NaOH aqueous solution, and the NH₄OH aqueous solution wereadded at 180 ml/hr, 180 ml/hr, and 10 ml/hr, respectively and reactedfor 24 hours to form a nickel-manganese-cobalt-based composite metalhydroxide as a precursor.

The precursor was mixed with lithium hydroxide as a lithium raw materialand tungsten oxide (WO₃) as a tungsten raw material such that the molarratio of M (the sum of Ni, Mn, and Co):Li:W was 0.995:2.0:0.005. Also,0.5 parts by weight of boron oxide (B₂O₃) was added as a sinteringadditive with respect to 100 parts by weight of the precursor. Then, alithium composite metal oxide was fabricated by performing a heattreatment for 10 hours at 820° C. under an oxygen atmosphere (oxygenpartial pressure 20%). The lithium composite metal oxide and distilledwater were mixed in a ratio of 1:1 and stirred for 10 minutes at a speedof 1,000 rpm, residual lithium on a surface was removed, and drying wasperformed for 12 hours in an oven at 150° C., thereby fabricating apositive electrode active material,(Li_(1.07)(Ni_(0.6)Mn_(0.2)Co_(0.2))_(0.995)W_(0.005)O₂), having amonolithic structure.

Example 1-2: Fabrication of Positive Electrode Active Material

In a batch-type 5 L reactor set at 60° C., nickel sulfate, cobaltsulfate, manganese sulfate, and magnesium sulfate were mixed in a molarratio of 60:20:20:0.2 in water to prepare a metal-containing solution ata concentration of 2M, and nickel sulfate, cobalt sulfate, manganesesulfate, and magnesium sulfate were mixed in a molar ratio of40:30:30:0.02 in water to prepare a second metal-containing solution ata concentration of 2M. A vessel containing the metal-containing solutionwas connected to enter the reactor, and a vessel containing the secondmetal-containing solution was connected to enter the metal-containingsolution vessel. In addition, a 4M NaOH solution and a NH₄OH aqueoussolution at a concentration of 7% were prepared and connected to thereactor, respectively.

3 L of deionized water was added to a coprecipitation reactor (capacity:5 L), the reactor was purged with nitrogen gas at a rate of 2 L/minuteto remove oxygen dissolved in water, and a non-oxidizing atmosphere wasformed in the reactor. Then, 100 ml of 4M NaOH was added thereto, andthe mixture was maintained at pH 12.0 at a stirring speed of 1,200 rpmat 60° C. Then, the metal-containing solution, the NaOH aqueoussolution, and the NH₄OH aqueous solution were added at 180 ml/hr, 180ml/hr, and 10 ml/hr, respectively and reacted for 30 minutes to form aseed of a hydroxide of a first metal-containing solution. Then, thesecond metal-containing solution was added to the first metal-containingsolution vessel at 150 ml/hr to induce growth of the hydroxide particleand induce formation of a concentration gradient inside the particle.Then, the reaction was maintained for 24 hours to grownickel-manganese-cobalt-based composite metal hydroxide. A resultantparticle of the nickel-manganese-cobalt-based composite metal hydroxideformed was separated, washed, and then dried in an oven at 120° C. tofabricate a precursor.

The resultant precursor formed was mixed with lithium hydroxide as alithium raw material and tungsten oxide (WO₃) as a tungsten raw materialsuch that the molar ratio of M(the sum of Ni, Mn, and Co)Mg:Li:W was0.975:0.02:2.0:0.005. Also, 0.5 parts by weight of boron oxide (B₂O₃)was added as a sintering additive with respect to 100 parts by weight ofthe precursor. Then, a lithium composite metal oxide was fabricated byperforming a heat treatment for 10 hours at 820° C. under an oxygenatmosphere (oxygen partial pressure 20%). The lithium composite metaloxide and distilled water were mixed in a ratio of 1:1 and stirred for10 minutes at a speed of 1,000 rpm, residual lithium on a surface wasremoved, and drying was performed for 12 hours in an oven at 150° C.,thereby fabricating a positive electrode active material,(Li_(1.07)(Ni_(0.6)Mn_(0.2)Co_(0.2))_(0.975)Mg_(0.02)W_(0.005)O₂),having a monolithic structure.

Comparative Example 1-1: Fabrication of Positive Electrode ActiveMaterial

In a batch-type 5 L reactor set at 60° C., nickel sulfate, cobaltsulfate, and manganese sulfate were mixed in a molar ratio of 60:20:20in water to prepare a metal-containing solution at a concentration of 2M. A vessel containing the metal-containing solution was connected toenter the reactor, and a 4M NaOH solution and a NH₄OH aqueous solutionat a concentration of 7% were prepared and connected to the reactor,respectively. 3 L of deionized water was added to a coprecipitationreactor (capacity: 5 L), the reactor was purged with nitrogen gas at arate of 2 L/minute to remove oxygen dissolved in water, and anon-oxidizing atmosphere was formed in the reactor. Then, 100 ml of 4MNaOH was added thereto, and the mixture was maintained at pH 12.0 at astirring speed of 1,200 rpm at 60° C. Then, the metal-containingsolution, the NaOH aqueous solution, and the NH₄OH aqueous solution wereadded at 180 ml/hr, 180 ml/hr, and 10 ml/hr, respectively and reactedfor 24 hours to form a nickel-manganese-cobalt-based composite metalhydroxide as a precursor.

The precursor was mixed with lithium hydroxide as a lithium raw materialsuch that the molar ratio of Li:M(the sum of Ni, Mn, and Co) was 1:1.07,and then a heat treatment was performed for 10 hours at 820° C. under anoxygen atmosphere (oxygen partial pressure 20%) to fabricate a positiveelectrode active material, (Li_(1.07)(Ni_(0.6)Mn_(0.2)Co_(0.2))O₂).

Comparative Example 1-2: Fabrication of Positive Electrode ActiveMaterial

The positive electrode active material,(Li_(1.07)(Ni_(0.6)Mn_(0.2)Co_(0.2))_(0.95)W_(0.05)O₂), was fabricatedby the same method as in Example 1-1 above except that the precursor,lithium hydrate as a lithium raw material and tungsten oxide (WO₃) as atungsten raw material were used such that the molar ratio of M(the sumof Ni, Mn, and Co):Li:W was 0.95:2.0:0.05 in Example 1-1 above.

Comparative Example 1-3: Fabrication of Positive Electrode ActiveMaterial

The positive electrode active material,(Li_(1.07)(Ni_(0.6)Mn_(0.2)Co_(0.2))_(0.995)W_(0.005)O₂), was fabricatedby the same method as in Example 1-1 above except that the heattreatment was performed for 10 hours at 1,050° C. in the presence ofboron oxide after the precursor was mixed with lithium hydroxide andtungsten oxide in Example 1-1 above.

Examples 2-1, 2-2 and Comparative Examples 2-1 to 2-3: Fabrication ofLithium Secondary Battery

A lithium secondary battery was fabricated using each of the positiveelectrode active materials fabricated in Examples 1-1, 1-2 andComparative Examples 1-1 to 1-3.

Specifically, each of the positive electrode active materials fabricatedin Examples 1-1, 1-2 and Comparative Examples 1-1 to 1-3, carbon blackwhich is a conductive material, and PVDF which is a binder were mixed ina weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent tofabricate a composition for forming a positive electrode (viscosity:5,000 mPa·s), and the composition was applied on an aluminum currentcollector, dried at 130° C., and rolled to fabricate a positiveelectrode.

Also, natural graphite as a negative electrode active material, carbonblack as a conductive material, and PVDF as a binder were mixed in aweight ratio of 85:10:5 in an N-methyl pyrrolidone (NMP) solvent tofabricate a composition for forming a negative electrode, and thecomposition was applied on a copper current collector to fabricate anegative electrode.

A porous polyethylene separator was interposed between the positiveelectrode and the negative electrode fabricated as above to fabricate anelectrode assembly, the electrode assembly was placed inside a case, andthen an electrolyte was injected into the case to fabricate a lithiumsecondary battery. Here, the electrolyte was fabricated by dissolvinglithium hexafluorophosphate (LiPF₆) at a concentration of 1.0M in anorganic solvent consisting of EC/DMC/EMC in a mixture volume ratio of3:4:3.

Experiment Example 1: Observation of Crystal Structure of PositiveElectrode Active Material

The positive electrode active material fabricated in Example 1-1 wasobserved with a scanning electron microscope, and a result thereof isshown in the FIGURE.

From the FIGURE, it can be confirmed that the positive electrode activematerial fabricated in accordance with Example 1-1 is a primary particlehaving a monolithic structure and a hexahedral shape, and a size thereofis uniform.

Experiment Example 2: Analysis of Concentration Gradient of MetalElements in Positive Electrode Active Material

To confirm a distribution of metal elements in a positive electrodeactive material particle fabricated in Example 1-2 above, etching wasperformed on the active material using HCl for various times, andelution amounts of the elements according to etching time or dissolutiontime were analyzed through inductively coupled plasma (ICP) analysis. Aresult thereof is shown in Table 1 below.

TABLE 1 Distance Dissolution from time particle Example 1-2(molar ratio)(minutes) surface(μm) Ni Co Mn Mg W 0 0 0.476 0.252 0.247 0.020 0.005 10.1 0.506 0.242 0.233 0.016 0.003 5 0.3 0.547 0.230 0.215 0.008 0 10 0.80.574 0.212 0.211 0.003 0 30 1.1 0.596 0.203 0.199 0.002 0 120 2.2 0.6120.196 0.192 0 0 (particle center)

Scanning positions in Table 1 above are shown in the FIGURE.

As a result of the experiment, it was confirmed that Ni, Co, and Mn areincluded with a concentration gradient in which a concentration of Nidecreases and concentrations of Co and Mn increase from the center ofthe active material particle toward the surface thereof. Mg was presentwith a concentration gradient that decreases from the surface of theparticle toward the center thereof.

Experiment Example 2: Analysis of Positive Electrode Active Material

An average particle size, a specific surface area, and a rolling densityof the positive electrode active material fabricated in Example 1-1 wasmeasured, and a result thereof is shown in Table 2 below.

(1) Average particle size (D₅₀): The positive electrode active materialparticle was introduced into a laser diffraction particle sizemeasurement device (for example, Microtrac MT 3000), and then anultrasonic wave of about 28 kHz was radiated with an output of 60 W tocalculate the average particle size (D₅₀) based on a particle sizedistribution at 50% in the measurement device.

(2) Particle size distribution value (Dcnt): a number average particlesize measured under 90% (Dn90), a number average particle size measuredunder 10% (Dn10), and a number average particle size measured under 50%(Dn50) of the positive electrode active material fabricated in Example1-1 were measured in an absorption mode using a Microtrac particle sizeanalyzer after the positive electrode active material fabricated inExample 1-1 was left in distilled water for 3 hours, and then theparticle size distribution value was calculated in accordance withEquation 1 below.

Dcnt=[Dn90−Dn10]/Dn50  [Equation 1]

(3) BET specific surface area: the specific surface area of the positiveelectrode active material was measured using the BET method, andspecifically, the specific surface area was calculated from a nitrogengas absorption amount under a liquid nitrogen temperature (77K) usingBELSORP-mini II of BEL Japan company.

(4) Rolling density: the rolling density was measured under a pressureof 2 tonf/cm² using a rolling density measuring device (HPRM-A1, HanTech Company Ltd.).

TABLE 2 Example Example Compar- 1-1 1-2 Compar- Compar- ative PrimaryPrimary ative ative Example 1-3 particle particle Example ExamplePrimary having having 1-1 1-2 particle Type mono- mono- Sec- Sec- havingParticle lithic lithic ondary ondary monolithic structure structurestructure particle particle structure Average 4.6 4.4 5.8¹⁾ 5.3¹⁾ 6.4particle size (D₅₀) (μm) Particle size 0.65 0.59 0.32 0.44 0.95distribution value (Dcnt) BET specific 0.15 0.21 1.45 1.84 0.12 surfacearea (m²/g) Rolling 3.28 3.16 2.24 2.07 2.87 density (g/cc) ¹⁾ is anaverage particle size of a secondary particle

Referring to Table 2, although each of the positive electrode activematerials of Example 1-1 and Example 1-2 were primary particles having amonolithic structure, the positive electrode active materials ofComparative Example 1-1 and Comparative Example 1-2 were secondaryparticles. It was confirmed that the positive electrode active materialsof Example 1-1 and Example 1-2 had a higher rolling density than thepositive electrode active materials of Comparative Example 1-1 andComparative Example 1-2 in the form of a secondary particle and had ahigher rolling density than the positive electrode active material ofComparative Example 1-3 in the form of a primary particle having amonolithic structure.

Experiment Example 3: Evaluation of Positive Electrode Active Material

A coin cell (negative electrode: Li metal) fabricated using each of thepositive electrode active materials fabricated in Example 1-1 andComparative Examples 1-1 to 1-3 was charged until a constant current(CC) of 0.1 C until 4.25 V at 25° C., and then charging at a constantvoltage (CV) of 4.25 V was performed, thereby performing one-timecharging until a charging current reached 0.05 mAh. Then, the coin cellwas left for 20 minutes and then discharged until 3.0 V at the constantcurrent of 0.1 C to measure a first-cycle discharge capacity. Then, eachof charge/discharge capacity, charge/discharge efficiency, and ratecapability was evaluated after changing a discharge condition to 2 C. Aresult thereof is shown in Table 3 below.

TABLE 3 First charge/discharge 2 C rate Charge Discharge Charge/ 2.0 C/capacity capacity discharge Capacity 0.1 C Type (mAh/g) (mAh/g)efficiency (%) (mAh/g) (%) Example 1-1 193.2 178.3 92.3 161.2 90.4Comparative 195.1 177.5 91.0 156.9 88.3 Example 1-1 Comparative 171.4154.6 90.2 139.3 90.1 Example 1-2 Comparative 190.5 168.4 88.4 142.084.3 Example 1-3

Referring to Table 3, a coin cell including the positive electrodeactive material of Example 1-1 exhibited further improvedcharge/discharge efficiency, capacity characteristics, and ratecapability than coin cells including the positive electrode activematerials of Comparative Example 1-1 to Comparative Example 1-3.

Experiment Example 5: Evaluation of Battery Characteristics of LithiumSecondary Battery

Battery characteristics of lithium secondary batteries (Example 2-1 andComparative Examples 2-1 to 2-3) respectively including the positiveelectrode active materials in Example 1-1 and Comparative Examples 1-1to 1-3 were evaluated using a method below.

Specifically, charge/discharge was performed for 300 times under a 1 C/2C condition within a driving voltage range of 2.8 V to 4.15 V at atemperature of 45° C. with respect to the lithium secondary battery.

As a result thereof, a cycle capacity retention rate, which is a ratioof a discharge capacity at the 300^(th) cycle to an initial capacityafter charge/discharge is performed 300 times at high temperature (45°C.) was measured and is shown in Table 4 below.

TABLE 4 300^(th) cycle capacity retention rate (%) at high Typetemperature (45° C.) Example 2-1 90.4 Comparative Example 2-1 82.7Comparative Example 2-2 67.1 Comparative Example 2-3 38.4

As a result of the experiment, it was confirmed that the lithiumsecondary battery using the positive electrode active materialfabricated in Example 2-1 exhibited superior output characteristics atroom temperature and high temperature and superior cycle characteristicsin comparison to Comparative Example 2-1 to Comparative Example 2-3.

1. A positive electrode active material for a secondary battery, thepositive electrode active material being a primary particle having amonolithic structure that includes a lithium composite metal oxide ofFormula 1 below, wherein the primary particle has an average particlesize (D₅₀) of 2 μm to 20 μm and a Brunauer-Emmett-Teller (BET) specificsurface area of 0.15 m²/g to 0.5 m²/g, and wherein the positiveelectrode active material has a rolling density of 3.0 g/cc or higherunder a pressure of 2 ton·f:Li_(a)Ni_(1-x-y)Co_(x) M1_(y) M3_(z) M2_(w)O₂  [Formula 1] in Formula 1,M1 is at least one selected from the group consisting of Al and Mn, M2is any one or two or more elements selected from the group consisting ofZr, Ti, Mg, Ta, and Nb, M3 is any one or two or more elements selectedfrom the group consisting of W, Mo, and Cr, and 1.0≤a≤1.5, 0≤x≤0.5,0≤y≤0.5, 0.005≤z≤0.01, 0≤w≤0.04, 0<x+y≤0.7.
 2. The positive electrodeactive material of claim 1, wherein in Formula 1, 0.4<x+y≤0.7.
 3. Thepositive electrode active material of claim 1, wherein at least onemetal element of nickel, M1, and cobalt exhibits a concentrationgradient that changes in the active material.
 4. The positive electrodeactive material of claim 1, wherein: nickel, M1, and cobaltindependently exhibit a concentration gradient that changes throughoutthe active material; the concentration of nickel decreases with aconcentration gradient in a direction from a center of the activematerial to a surface thereof; and the concentrations of cobalt and M1independently increases with a concentration gradient in the directionfrom the center of the active material to the surface thereof.
 5. Thepositive electrode active material of claim 1, wherein the M1 ismanganese (Mn).
 6. The positive electrode active material of claim 1,wherein the positive electrode active material has a polyhedral shape.7. The positive electrode active material of claim 1, wherein thepositive electrode active material has a particle size distributionvalue (Dcnt), which is defined by Equation 1 below, of 0.5 to 1.0.Dcnt=[Dn90−Dn10]/Dn50  [Equation 1] (In Equation 1, Dn90, Dn10, and Dn50are number average particle sizes measured under 90%, 10%, and 50%,respectively)
 8. A method of fabricating the positive electrode activematerial for a secondary battery of claim 1, the method comprising: astep of preparing a precursor by mixing a nickel raw material, a cobaltraw material, and an M1 raw material, wherein M1 is at least one elementselected from the group consisting of Al and Mn, and then performing areaction; a step of mixing the precursor with a lithium raw material andan M3 raw material, wherein M3 is any one or two or more elementsselected from the group consisting of W, Mo, and Cr, such that a molarratio of Li/Me is 2.0 or higher, wherein Me is the sum of metal elementsin the precursor and the element M3, and then sintering at 700° C. to900° C. in the presence of a boron-based sintering additive; and a stepof washing a product obtained by a result of the sintering such that amolar ratio of Li/Me′ in the finally fabricated positive electrodeactive material is from 1.0 to 1.5, wherein Me′ is the sum of metalelements, excluding lithium, in the positive electrode active material,and then drying at 150° C. to 400° C.
 9. The method of claim 8, whereinan M2 raw material is further added in the preparing of the precursor orthe sintering, wherein M2 is any one or two or more elements selectedfrom the group consisting of Zr, Ti, Mg, Ta, and Nb.
 10. The method ofclaim 8, wherein the precursor is fabricated by adding an ammoniumcation-containing complexing agent and a basic compound to ametal-containing solution, which is produced by mixing the nickel rawmaterial, the cobalt raw material, and the M1 raw material andperforming a coprecipitation reaction.
 11. The method of claim 10,wherein a second metal-containing solution including the nickel rawmaterial, the cobalt raw material, and the M1 raw material in differentconcentrations from the metal-containing solution is further added tothe metal-containing solution.
 12. The method of claim 8, wherein theboron-based sintering additive includes any one or two or more selectedfrom the group consisting of boric acid, lithium tetraborate, boronoxide, and ammonium borate.
 13. A positive electrode for a secondarybattery, the positive electrode comprising the positive electrode activematerial of claim
 1. 14. A lithium secondary battery comprising thepositive electrode of claim 13.